Load current measurement is crucial in many applications such as torque control with electric motors, the control of brushless DC motors or similar applications. As other inductive loads, electric motors are often driven by one or more transistor half-bridges. Two half-bridges may be connected to form an H-bridge three half-bridges may be connected to form a three-phase transistor bridge to drive the three phases of a three-phase electric motor. The half-bridges are usually driven in accordance with a pulse-width modulated control signal. However, other modulation schemes may be applicable instead of pulse-width modulation (PWM).
A direct load current measurement at the inductive load (e.g. by connecting a shunt resistor in series to the load) is often avoided as the resulting current sense signal is subject to rather high and abrupt variations of the common mode (CM) voltage. To avoid such high CM variations, current sensing is often done at the low-side switch of a transistor half-bridge. Such an arrangement, however, allows only for a current measurement while the low side switch is on. As a consequence, the current sense signal is discontinuous, which is undesired in some applications. When sensing the current directly at the load, the current sense signal is continuous. However, the mentioned CM variations cause undesired voltage spikes (due to parasitic inductances), which may have a negative impact on the achievable accuracy. Although the spikes can be reduced using filters, the mentioned CM variations remain an issue. Thus, there is a need for a current sense circuit with a high CM rejection.